Sound absorbing device of the type adapted to cover the ears of a user

ABSTRACT

A sound absorbing device of the type adapted to cover the ears of a user and comprising a sound absorbing material, wherein the sound absorbing material comprises of a thixotropic material.

FIELD OF THE INVENTION

The present invention relates to a sound absorbing device of the typeadapted to cover the ears of a user for example noise-cancellingheadphones, ear plugs, or ear muffs.

BACKGROUND TO THE INVENTION

It is commonly known in the art that the ambient or background noiselevels experienced by people every day may be both harmful anddistracting. The noise of burglar alarms, jackhammers, buses, trains,heavy commuter traffic and construction sites in large city and urbanareas can aggravate residents and commuters alike.

Users of portable music players such as MP3 players generally have thevolume of their headphones raised to “block out” the ambient noise ofthe environment they find themselves in. Long-term use of such musicplayers at high volumes can damage hearing. Furthermore, listening tomusic players at high levels when walking or cycling through citycentres and urban areas inhibits other senses from functioning optimallysuch as being aware of traffic, other cyclists and pedestrians, which isa key sense to have functioning optimally in a busy city centre wheretraffic is a potential, and sometimes fatal, hazard.

Furthermore, in industry the welfare of workers who are exposed tolevels of excessive noise in the workplace is important. The use of earmuffs, ear plugs and the like are commonly required by those workers toprotect their hearing. However, the wearing of such ear protectors mayalso inhibit the wearer's awareness of their surroundings which couldlead to accidents occurring in the workplace.

The use of materials in energy absorbing compositions is known, forexample as disclosed in WO 2007/079320 (Sereboff). The composition is asubstantially non-elastic incompressible composition, which does notquickly self-level under standard operating conditions and provides anincident energy absorbing property, wherein the incident energy mayinclude sound energy. The applicants of WO 2007/079320 also claim thatthe composition may be used as an effective barrier against lowfrequency, high-energy sound in, for example, headphone housings,speaker housings, sound-proofing in walls or submarine engine rooms.However, the compositions of WO 2007/079230 are restricted to Binghammaterials, which will not flow until stress can be applied and a certainvalue, the yield stress value, is reached. Beyond this point the flowrate increases steadily with increasing shear stress. In essence aparticular minimum value has to be achieved to activate the material.

Thixotropic materials can be composed of a number of combinations, someof which are listed in this document and can be tailored to specificfrequency and decibel adapted limits but all would be structured to suitbiological protection from an early sound level stage while stillallowing normal conversation and safety discussion to be maintainedbetween co-workers when on-site. As well as this, immediate activationby pre-agitation or loud sound induction would make for a highlyeffective responsive invention.]

Dilitant materials are also known from the prior art as being used inhearing protection ear cups where external noise is prevented frompassing directly through the ear cup itself (US 2003/0034198).Furthermore, the use of particles with mis-matched characteristicacoustic impedances embedded within a matrix of a material that cansupport shearing loads, propagates energy diffusion thereby providingacoustic and vibration damping (U.S. Pat. No. 5,400,296). However,dilitant materials become denser when exposed to high shear/mechanicalforces such as energy waves. The denser the material, the easier it isto conduct the vibrations and hence the sound travels faster and moreefficiently. This can be seen clearly in a simple dilitant material suchas corn flour and water. When left in a container, it is liquid butattempt to stir it and it hardens up. If you go so far as to try andlift it out, it will go solid and as long as you continually move itbetween your hands, it will stay solid but the moment you leave italone, it will return to liquid form and drip through your fingers. Assuch, dilitant materials are not effective sound absorbing materials dueto the fact that they harden when exposed to energy waves.

A protective helmet comprising a rigid shell, an inner lining with atleast one inflatable cell and inflation/deflation means is known fromthe prior art, as described in UK Patent Application No. GB 2 340 281.The inflatable cell may be filled with a fluid having thixotropicproperties using a manual or electronic-powered pump. A method forobtaining rubber ear plugs is described in U.S. Pat. No. 3,782,379,where vulcanizable rubber is used at a certain viscosity to providethixotropic properties.

There is a need therefore to provide inner and external ear protectorsand/or inner- and outer-ear headphones which do not impact on the user'sawareness of their surroundings while at the same time reducing oreliminating external ambient noise. The present invention intends toaddress at least one of the problems mentioned above.

SUMMARY OF THE INVENTION

The invention is based on the use of a thixotropic material as a soundabsorbing medium in devices which are adapted to attach to or cover auser's ear. Examples of such devices would be noise protectionheadphones, audio headphones, and ear plugs. The thixotropic materialhas a resting viscosity which decreases as sound energy is incident onthe material. As the viscosity of the material decreases, the level ofattenuation of the incident sound increases. The thixotropic materialreacts in such a way as to transfer some of the entering sound energyinto kinetic energy that changes the structure of the thixotropicmaterial and lessens the amount of sound energy passing through thematerial. Thus, when such a material is employed in noise protectionheadphones/earmuffs, the level of attenuation of sound will increase asthe intensity of the sound increases, thereby allowing a user to hearlow intensity sounds such as conversation though the headphones (whenthe material is at or close to a resting viscosity), while attenuatinghigh intensity sounds.

Accordingly, in a first aspect, the invention provides a sound absorbingdevice of the type adapted to cover the ears of a user and comprising asound absorbing material contained within a container in the form of acellular scaffold, wherein the sound absorbing material comprises of athixotropic material which is located in the cells of the cellularscaffold.

Ideally, the sound absorbing material may be enclosed within at leastone container, and wherein the container may be expandable to allow anincrease in volume of the sound absorbing material.

In one aspect of the invention, the cellular scaffold has a honeycombstructure typically having a plurality of cells arranged in a honeycombstructure, in which the thixotropic material is contained within thecells. Ideally, the cellular scaffold is in the form of a polymer filmhaving cellular compartments or pockets, wherein the sound absorbingmaterial is located within the cellular compartments.

Alternatively, the cellular scaffold may comprise at least one tubehaving first and second ends and a thixotropic material-containing lumenextending between the ends. Optionally, the tube may be disposed withinthe sound absorbing device such that at least one end of the tube facestowards incident sound. Typically, the cellular scaffold may comprise aplurality of tubes.

Ideally, the or each tube may be U-shaped and in which both ends of thetube preferably face towards incident sound. The or each tube mayfurther comprise an additional tube disposed within the U-shaped tubebetween the first and second ends, wherein the additional tube has anopen end which faces towards the incident sound.

In one embodiment of the present invention, one end of each of aplurality of tubes may be disposed within a base. In a furtherembodiment, the present invention may further comprise a plurality ofsubstantially U-shaped tubes in which both ends of the tubes aredisposed in the base.

In one embodiment of the present invention, at least two and preferablyat least three (for example from 2 to 4 or 5 layers) layers of honeycombstructure may be arranged in a facing relationship, wherein the at leasttwo layers are ideally disposed such that the cells of the honeycombstructure of a first layer are not in register with the cells of thehoneycomb structure of the second layer. Preferably, the structure ismade up of three layers oriented at 120° apart. The layers of honeycombstructure may be separated by a sound insulating layer. A soundinsulating layer may be any sound insulating layer known to a person ofordinary skill in the art, such as a fluid, a gas, a polymer, and thelike. The arrangement of the layers of honeycomb structure in thisembodiment results in a Helmholtz resonance effect, that is, increasingsound absorption through differential air pressure between the out ofregister arrangement of the cells in each layer.

It should be understood that other embodiments of the cellular scaffolddescribed herein can be arranged in layers and disposed relative to eachother such that the cells of the cellular structure are not in registerand separated by a sound insulating layer.

Ideally, the invention provides a sound absorbing device of the typeadapted to cover the ears of a user and comprising a sound absorbingmaterial, wherein the sound absorbing material comprises a thixotropicmaterial.

The thixotropic material may be a fluid, a solid or semi-solid materialsuch as a gel or resin. In a preferred embodiment of the invention, thethixotropic material is a liquid. Examples of thixotropic materials willbe well known to those skilled in the field. Examples would includestructured liquids, suspensions, emulsions, polymer solutions, aqueousiron oxide gels, vanadium pentoxide sols, starch pastes, pectin gels,flocculated paints, clays, soil suspensions, creams, drilling muds,flour doughs, flour suspensions, fibre greases, jellies, paints, honey,carbon-black suspensions, hydrophobically modified hydroxethylcellulose, non-associative cellulose water solutions, flocculatedpolymer latex suspension, rubber solutions, metal slushes, bentoniteclays, modified laponites, oils, lubricants, coal suspensions, xanthangums, organic bentonite, fumed silica, aluminum stearate, metal soap,castor oil derivatives or thixotropic epoxy resin without prejudice orexclusion of like materials to these listed. The thixotropic materialcan be found to be effective in systems containing non-sphericalparticles, and is also associated with certain colloids which form gelswhen left to stand but become sols when stirred or shaken. They can alsobe associated with concentrated solutions of substances of highmolecular weight colloidal suspensions.

Typically, the sound absorbing material consists essentially of athixotropic material, especially a thixotropic liquid or gel.

In a flocculated system, the microstructure at rest can be seen to be aseries of large floccules. A floccule is a small loosely aggregated massof flocculent material suspended in or precipitated from a liquid. If anapplied shear rate is given with an appropriate time interval, thefloccule disintegrates into its constituent primary particles. Theminimum viscosity can be seen with individual particles. Individualparticles are considered to be those of simplest primary structure wherethe flocculated system has degraded to such a state as to contain thesmallest possible particles within a specified shear range and withoutrisking the integrity of the structure holding the material. In anyflocculated system, the disintegration will be tending towards anequilibrium scenario that is held by hydrodynamic stresses pullingstructures apart by erosion and upon removal of the appliedstress/strain, that is, Brownian and shear forces rebuilding thestructure by collision and accumulation of particles. In thisflocculated structure the forces holding the structure together arecolloidal in design and act over approximate distances of 10 nanometres,but may vary according to requirement.

Diffusion rates of isolated floccules decrease significantly as theirrespective size increases. There is a simple inverse relationshipbetween particle size translational diffusion, which is demonstrated byEinstein's translational coefficient.

Shear thinning systems can occur due to loss of association in polymersolutions, rod-like alignment of particles in the direction of flow,microstructure rearrangement or flocculation disintegration.

Typically, the sound absorbing material is enclosed within a container,wherein the container is expandable to allow an increase in volume ofthe sound absorbing material. In a preferred embodiment of theinvention, the sound absorbing device of the invention is constructed toallow shear be applied by a user to the thixotropic material containedtherein, thereby activating the material by reducing its viscosity andincreasing its sound absorbing capacity.

Ideally, the container comprises a cellular scaffold, and wherein thesound absorbing material is located in the cells of the cellularscaffold. Suitably, the cellular scaffold is formed of a thixotropicmaterial.

Various types of containers are envisaged. Suitably, the containercomprises a polymeric pouch which contains the sound absorbing material,wherein the polymeric pouch is expandable in response to pressureexerted by the thixotropic material as it decreases in viscosity andincreases in volume. In another embodiment, the container comprises atube having a first end, a second end, and a lumen extending between thefirst and second ends adapted for containing the thixotropic material,and wherein the tube is preferably disposed within the sound absorbingdevice such that at least one end of the tube faces towards incidentsound (i.e. sound from the external environment). In a particularlypreferred embodiment, a plurality of tubes are provided and ideallydisposed in an interleaving arrangement. Thus incident sound enters eachtube at one end and travels along the tube towards the second end. Inanother embodiment, the tube is curved, ideally u-shaped, in which casethe tube is optionally disposed within the sound absorbing device suchthat both ends of the tube face towards incident sound, although thetube may be disposed such that both ends of the tube do not face towardsincident sound, for example they could face away from incident sound.Ideally, a series of curved or U-shaped tubes are provided.

In this specification, the term cellular scaffold should be understoodto mean a scaffold or substrate having a plurality of pockets orcontainers or compartments or tubes adapted to hold the thixotropicmaterial. One example of a cellular scaffold would be a honeycombstructure, which may be formed using conventional moulding techniquesfrom a polymeric or resin material. Another example of a cellularscaffold would be a polymer film having cellular compartments, whereinthe sound absorbing material is located within the cellularcompartments. Examples of such polymeric cellular scaffolds would bebubble wrap or polymer ice-cube making bags. In both cases, thethixotropic material would be disposed within the pockets of cells ofpolymer film. Another example of a cellular scaffold would be a seriesof tubes, suitably cylindrical tubes, and ideally U-shaped tubes.

Ideally, the thixotropic material is a material which exhibits a lowviscosity drop in response to low intensity noise and disproportionatelyhigh viscosity decrease in response to high intensity noise. Examples ofsuitable materials include structured liquids, suspensions, emulsions,polymer solutions, aqueous iron oxide gels, vanadium pentoxide sols,starch pastes, pectin gels, flocculated paints, clays, soil suspensions,creams, drilling muds, flour doughs, flour suspensions, fibre greases,jellies, paints, honey, carbon-black suspensions, hydrophobicallymodified hydroxethyl cellulose, non-associative cellulose watersolutions, flocculated polymer latex suspension, rubber solutions, metalslushes, bentonite clays, modified laponites, oils, lubricants, coalsuspensions, xanthan gums, organic bentonite, fumed silica, aluminumstearate, metal soap, castor oil derivatives or thixotropic epoxy resin.The use of a semi-liquid gaseous phase thixotropic combination which maytend towards gaseous state by agitation from the applied shear rate isalso envisaged. This may also be achieved by partially evacuating thechamber in which it is disposed.

In a second aspect, the invention relates to a sound absorbing deviceaccording to the invention in the form of an ear plug, an audioheadphone, an audio ear bud system, or a noise protection headphone.

Generally, in audio headphones and audio ear bud systems of theinvention, the sound absorbing material is disposed between the speakerand the external environment, and within the ear buds, respectively.Thus, for example, the thixotropic material may form a barrier layerthat is disposed in the headphone cup such that it covers the ear of auser, or is disposed within the ear buds to provide a barrier betweenthe ear and the external environment. In this embodiment, the purpose ofthe thixotropic material is to dampen external sounds, thereby allowinga user to hear more clearly the sounds being produced by the speaker inthe headphone/audio ear bud system.

Generally, for noise protection headphones, the sound absorbing materialis disposed within the headphone to provide a barrier between the earand the external environment.

Preferably, the cup of the headphone is a soft, deformable constructionallowing a user to apply shear to the thixotropic material containedwithin the headphone.

Typically, an audio ear bud or ear plug of the invention is formed of athixotropic material. Thus, the audio ear bud or ear plug may be formedof a moulded resinous thixotropic material. In another embodiment, theaudio ear bud or ear plug may comprise an external shell and an internalcavity (container), wherein the thixotropic material is disposed withinthe cavity. The cavity may contain a container for holding thethixotropic material, for example a cellular scaffold as describedabove. Ideally, the ear plug or audio ear bud comprises a soft,deformable, material and allows the shear to be applied to the ear plugor audio ear bud by, for example, compressing or kneading the earplug oraudio ear bud.

In a third aspect, the invention relates to a method of protecting theears from high intensity noise comprising the step of placing a soundabsorbing device of the invention over the ears, wherein the thixotropicmaterial has a resting viscosity which exhibits low resistance topassage of low intensity noise, and wherein the thixotropic materialdecreases in viscosity in response to incident high noise to therebyexhibit high resistance to the high intensity noise.

In one embodiment of the invention, the method involves a step ofapplying shear to the thixotropic material in the sound absorbing deviceto increase the resistance to high intensity noise. In this way, forexample, a user can control the noise resistance of the device. Thus,when a user of, for example, noise protection headphones is about toenter a noisy area of a processing plant, for example, they could removethe headphones and shake them to apply shear to the thixotropic materialand thereby increase the noise protection, and then put the headphoneson prior to entering the noisy area of the processing plant. Othermethods of applying shear would be to, for example, massaging thethixotropic material. For audio ear buds or ear plugs of the invention,especially audio ear buds or ear plugs that are resiliently deformable,the ear bud or ear plug may be squeezed or kneaded to “activate” thethixotropic material. For headphones of the invention, the cups of theheadphones may also comprise a construction which allows the thixotropicmaterial contained therein to be squeezed or kneaded to activate thematerial.

The invention also relates to a sound absorbing material comprising acellular scaffold in which the cells of the scaffold contain athixotropic material. Typically, the cellular scaffold takes the form ofa honeycomb structure in which the voids of the honeycomb structurecontain a thixotropic material, ideally a thixotropic fluid. Thus, thesound absorbing material ideally comprises a layer of honeycombstructure having top and bottom seal layers which seal the voids in thehoneycomb structure. In another embodiment, the cellular scaffoldcomprises a polymeric film having a multiplicity of cells, in which thecells contain the thixotropic material. This may take the form of astructure similar to layers of ice cube sheets interleaved or smallerdimensional rounded pockets that fit between each layer's cavity, i.e.one pocket would fit into the inner pocket space of another sheet and soon.

The honeycomb structure is seen as being potentially advantageous from astructural integrity and sound dampening combination allowing individualcompartments but with the added advantage of strength and minimumnon-thixotropic surface area.

The polymeric film in the form of a disc may be stacked one on top ofthe other. The discs may comprise a series of honeycomb structures,where each honeycomb structure may encase or may be comprised of athixotropic material.

In a further embodiment of the invention, the thixotropic material maybe selected to absorb sound energy at a specific frequency range. Forexample, the thixotropic matrix may comprise materials that respondmaximally to the increasing noise levels at specific frequency ranges.In one embodiment, the thixotropic material provides hearing protectionover a range of about 20 Hz to about 20000 Hz (20 KHz) of a human'shearing range. In another embodiment, the thixotropic material provideshearing protection for ultrasound (greater than 20 KHz) and infrasound(less than 20 Hz) as well as customised intermediate audible ranges forthe purpose of particular environments such as rifle ranges, concerts,construction sites etc. For example in the area of ballistics in riflesand cannons, it is typically in the lower audible range of below 20 Hzbut construction sites may be in the higher and lower ranges with highspeed drills and low frequency pile drivers. Within the audible ranges,decibel is the main factor outside of the resonant range of 2 KHz to 4KHz. There may be instances when infrasound/ultrasound can cause issuesthat are not audible to the human senses but can cause structuralintegrity problems. For example, in ultrasonic welding at frequenciesfrom 20 KHz to 40 KHz, shielding may be required around the non targetedareas. In non-destructive testing of material flaws in ranges up to 10MHz, again the non-desired target area may be shielded. There may alsobe issues surrounding animal welfare in ultrasound and infrasoundenvironments. It is known to those skilled in the art as to thesensitivities of certain animals to ultrasound (bats, dogs, rodents,dolphins, whales, fish, cattle and horses etc).

Ultrasonic sound energy has potential physiological effects: it maycause an inflammatory response and the unwelcome heating effect of softtissue. Ultrasound sound energy can also produces a mechanicalrarefaction/compression wave through soft tissue. This pressure wave hasthe potential of causing microscopic bubbles in living tissues and canlead to the distortion of the cell membrane which affects intracellularactivity. Ultrasound causes molecular friction and heats the tissues inthe body. This effect is usually minor as normal tissue perfusiondissipates most of the heat, but with high enough intensity, it cancreate small pockets of gas in body fluids or tissues which may expandand contract. This phenomenon can be called cavitation. It may be seenthen that a need for thixotropic energy absorbance could be utilised inmedical environments for shielding either from diagnostic, medical,surgical or dentistry purposes. In each of these cases narrow ranges maybe isolated to allow for specific shielding for specific purposes. Forexample, when utilising ultrasonic surgery in the regions of 250 KHz to2 MHz, only the desired body area should be exposed to the full strengthof energy so as to lower the hazards attached to the non-affected part.

In the case of most high noise risks, the frequency range is quite lowand could be simple to have one thixotropic material. It may require asecond thixotropic material which deal with higher frequency noises suchas high speed drills or alarms.

In one embodiment of the invention, the thixotropic material may beselected from the group comprising structured liquids, suspensions,emulsions, polymer solutions, aqueous iron oxide gels, vanadiumpentoxide sols, starch pastes, pectin gels, flocculated paints, clays,soil suspensions, creams, drilling muds, flour doughs, floursuspensions, fibre greases, jellies, paints, honey, carbon-blacksuspensions, hydrophobically modified hydroxethyl cellulose,non-associative cellulose water solutions, flocculated polymer latexsuspension, rubber solutions, metal slushes, bentonite clays, modifiedlaponites, oils, lubricants, coal suspensions, xanthan gums, organicbentonite, fumed silica, aluminum stearate, metal soap, castor oilderivatives or thixotropic epoxy resin. The thixotropic material may beused as a sound absorbing material for the following applications:

-   1) Filling for ear plugs;-   2) Interior of ear muff whether as a single layer, multiple layers    or a tubular structure;-   3) Outer interface for a noise cancelling headphone set or audio ear    bud system to lessen ambient noise to user; and-   4) Interior of an ear band whether as a single layer, multiple    layers or tubular structure.

The advantages of such a system are:

-   1) No electronics or power sources are required;-   2) The at least one thixotropic layer will respond proportionately    to the amount of sound energy entering, so as to allow the user to    hear voices or warnings but further dampen any extremely loud    noises, for example, machinery such as drills, pile drivers, guns,    house alarms, trains, and the like.-   3) The material is cheap to produce and manufacture into existing    headphone/ear muff/ear plug designs.-   4) The response rate of the material will not require a yield stress    rate such as that required for Bingham fluids, and hence will work    quicker and be able to be pre-agitated before entering the    environment.-   5) The main difference between linear viscoelasticity and thixotropy    are that the former remains unchanged due to elasticity in the    linear region and the latter breaks down in structure, even though    both materials are time dependent.

Thus, the sound absorbing device of the present invention differs fromconventional sound absorbing devices in that it is particularlyconcerned with thixotropic materials which attribute a decrease of theapparent viscosity under constant shear stress or shear rate,immediately followed by a gradual return to equilibrium once the shearstress or strain is removed. It is dependent on the finite time takenfor the shear induced structural change in the thixotropic materialmicrostructure caused by the stress tearing and flow induced collisions.Once flow ceases, Brownian motion can return the elements of themicrostructure back to a more favourable equilibrium once again. It is areversible process determined by time and shear stress or strain.

In terms of preventing hearing damage, time is as important a concern asamplitude. The human hearing system is able to deal with particularfrequency and amplitude combinations for finite periods of time. Sincethixotropic compositions act immediately and progress in loweringviscosity as time passes under shear rate, the protection is increasedfor the user the longer they remain in a high-risk sound environment.This allows for the use of this invention in ear plugs designed forconcert audiences who wish to hear the music or performance but atincreasingly safe limits as well as all other listed uses forconstruction workers, security personnel, dog handlers, dentists,firearms users, military personnel and associated sound induced traumarisk environments.

The thixotropic material can be comprised of a variety of materials,comprising existing thixotropic liquids and resins or a combinationthereof structured liquids, suspensions, emulsions, polymer solutions,aqueous iron oxide gels, vanadium pentoxide sols, starch pastes, pectingels, flocculated paints, clays, soil suspensions, creams, drillingmuds, flour doughs, flour suspensions, fibre greases, jellies, paints,honey, carbon-black suspensions, hydrophobically modified hydroxethylcellulose, non-associative cellulose water solutions, flocculatedpolymer latex suspension, rubber solutions, metal slushes, bentoniteclays, modified laponites, oils, lubricants, coal suspensions, xanthangums, organic bentonite, fumed silica, aluminum stearate, metal soap,castor oil derivatives or thixotropic epoxy resin. The particularconcentrations have yet to be determined but may comprise pre-existingsound insulation resins or solutions or newly tailored mixtures. Thethixotropic material may be injected into a cavity in earplugs and/orear muffs.

The louder the sound input the thixotropic material becomes less denseand resists conducting the sound to the ear. In addition to this, whatremaining sound is left will find it harder to pass through a nowsignificantly less dense material. But if the sound is very quiet, itwill allow it to pass. This then creates a set of hearing protection earmuffs that allow conversation but instantly react to loud noises ofvarying frequencies and levels as soon as they try and enter the ear.

The inclusion of a harder thixotropic material in a cavity for ear muffsor headphones would also be easier than trying to inject a dilitantmaterial in less viscous form. An expansion area in the cavity wouldallow the material to change state successfully and not rupture orpressurise the cavity so as to restrict change. Such an applicationcould be used in noise cancelling headphones so as to further restrictexternal noise and allow the user to listen to their music at lowervolumes than previously experienced.

Definitions

It is to be taken in this specification that the term “thixotropicmaterial” refers to a material that has a certain viscosity in a restingstate, but which changes viscosity in response to shear. Thixotropicmaterial may take the form of solids, liquids, gases, and semi-solidmaterials. Examples of thixotropic materials include structured liquids,suspensions, emulsions, polymer solutions, aqueous iron oxide gels,vanadium pentoxide sols, starch pastes, pectin gels, flocculated paints,clays, soil suspensions, creams, drilling muds, flour doughs, floursuspensions, fibre greases, jellies, paints, honey, carbon-blacksuspensions, hydrophobically modified hydroxethyl cellulose,non-associative cellulose water solutions, flocculated polymer latexsuspension, rubber solutions, metal slushes, bentonite clays, modifiedlaponites, oils, lubricants, coal suspensions, xanthan gums, organicbentonite, fumed silica, aluminum stearate, metal soap, castor oilderivatives or thixotropic epoxy resin. The term should be taken toinclude thixotropic materials which show a time-dependent change inviscosity, i.e. the longer the fluid/material undergoes shear stress,the lower its viscosity. Many gels and colloids are thixotropicmaterials which exhibit a stable form at rest but become fluid whenagitated. Typically the thixotropic material has a dynamic viscosity offrom 10⁻³ to 10³ Pa/s, preferably 10⁻² to 10² Pa/s, and ideally 2 Pa/sto 250 Pa/s, when measured using a Rotating Cylinder Viscometer method(European Pharmacopoeia 5.0 2.2.10 (January 2005)). For example, if aliquid (or solid, gas and semi-solid material) is placed in viscometerand a solid object, such as a polyamide rotor, is immersed in the liquidand rotated at a constant speed around its central axis, the rotor willexperience a retarding force due to the viscous drag of the liquid. Byknowing the dimensions of the viscometer, the viscosity of the liquidcan be calculated. The viscosity of non-Newtonian systems may also bemeasured using a Rotating Cylinder Viscometer by obtaining two shearrates and interpolating the readings. In a preferred embodiment, thethixotropic material has a viscosity in the range of honey (preferablyBoyne Valley Honey (Ireland)) and peanut butter (preferably Panda smoothpeanut butter (Boyne Valley Group, Ireland)).

The cellular scaffold of the present invention, or part thereof, forexample the part of the scaffold which holds the tubes, or indeed thetubes themselves, may be composed of, for example, Sheetrock, MassLoaded Vinyl, Hardwood, rubber, cork, fibreboard, wood wool cement,glass silk, mineral wool, acoustic foam, sponge, acoustic tile, glassfibre, porous plastic, porous rubber, rubber foam, melamine sponge,foam, rubber, latex, porous absorbers, and aerated plaster. It shouldalso be taken in this specification that the cellular scaffold may becomposed of a thixotropic material described above.

In the specification the terms “comprise, comprises, comprised andcomprising” or any variation thereof and the terms “include, includes,included and including” or any variation thereof are considered to betotally interchangeable and they should all be afforded the widestpossible interpretation and vice versa.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more clearly understood from the followingdescription of an embodiment thereof, given by way of example only, withreference to the accompanying drawings, in which:

FIG. 1 illustrates a first honeycomb layer structure for a soundabsorbing device of the present invention;

FIG. 2 is an exploded view of a thixotropic device of the presentinvention comprising a honeycomb lattice as illustrated in FIG. 1;

FIG. 3 illustrates a second honeycomb layer structure for a soundabsorbing device of the present invention;

FIG. 4 illustrates a third honeycomb layer structure for a soundabsorbing device of the present invention;

FIG. 5 illustrates a cylindrical tubular structure for a sound absorbingdevice of the present invention;

FIG. 6 illustrates a multiple interleaved cylindrical tubular structurefor a sound absorbing device of the present invention;

FIG. 7 illustrates (A) a side view and (B) a perspective view of asubstantially U-shaped structure for a sound absorbing device of thepresent invention;

FIG. 8 illustrates a set of headphones comprising a sound absorbingmaterial enclosed in a container of the present invention;

FIG. 9 illustrates a fourth honeycomb layer structure for a soundabsorbing device of the present invention;

FIG. 10A-10D illustrates in more detail the substantially U-shapedstructure of FIG. 7 in (A) front, (B) plan, (C) perspective and (D) sideviews;

FIG. 11A-11D illustrates a second substantially U-shaped structure for asound absorbing device of the present invention in (A) plan, (B) front,(C) side and (D) perspective views;

FIG. 12 illustrates a third substantially U-shaped structure for a soundabsorbing device of the present invention in (A) plan, (B) front, (C)side and (D) perspective views;

FIG. 13 illustrates a multiple cylindrical tubular structure for a soundabsorbing device of the present invention in (A) side, (B) elevation,(C) plan and (D) perspective views;

FIG. 14 illustrates a second multiple cylindrical tubular structure fora sound absorbing device of the present invention (A) side, (B)elevation, (C) plan and (D) perspective views;

FIG. 15 illustrates a graph of dB Change vs. Frequency (Hz) for oneembodiment of the present invention as shown in FIG. 11 and the industrystandard 3M® headphone;

FIG. 16 illustrates a graph of dB Change vs. Frequency (Hz) for oneembodiment of the present invention as shown in FIG. 3 and the industrystandard 3M® headphone;

FIG. 17 illustrates a graph of dB Change vs. Frequency (Hz) for oneembodiment of the present invention as shown in FIG. 13 and the industrystandard 3M® headphone;

FIG. 18 illustrates a graph of dB Change vs. Frequency (Hz) for oneembodiment of the present invention as shown in FIG. 14 and the industrystandard 3M® headphone;

FIG. 19 illustrates a graph of dB Change vs. Frequency (Hz) for oneembodiment of the present invention as shown in FIG. 1 and the industrystandard 3M® headphone;

FIG. 20 is a schematic diagram of the testing environment used todetermine the dB Change vs. Frequency (Hz) as shown in FIGS. 15 to 19.

DETAILED DESCRIPTION OF THE DRAWINGS

The invention is based on the use of a thixotropic material as a soundabsorbing medium in devices which are adapted to attach or cover auser's ear. Examples of such devices would be noise protectionheadphones, audio headphones, and ear plugs. The thixotropic materialhas a resting viscosity which decreases as sound energy is incident onthe material. As the viscosity of the material decreases, the level ofattenuation of the incident sound increases. Thus, when such a materialis employed in noise protection headphones/earmuffs, the level ofattenuation of sound will increase as the intensity of the soundincreases, thereby allowing a user to hear low intensity sounds such asconversation though the headphones (when the material is at or close toa resting viscosity), while attenuating high intensity sounds.

Referring now to the figures, where FIG. 1 illustrates a generalembodiment of a sound absorbing device of the present invention.Specifically, FIG. 1 illustrates a perspective view of a sound absorbingdevice of the present invention, which in this instance is configured asa honeycomb layer and is generally referred to by reference numeral 1.The honeycomb layer 1 is arranged in a sheet 2 and is formed frompolyethylene plastic foil. The sheet 2 comprises a series of rows 3 andcolumns 4 of a honeycomb structure or cells 5. The cells 5 are filledwith a thixotropic material, in this case lubricating oil. Thethixotropic material reacts in such a way as to transfer some of theentering sound energy into kinetic energy that changes the structure ofthe thixotropic material and lessens the amount of sound energy passingthrough the material. In a further embodiment, the honeycomb cells 5 arecomprised of the thixotropic material. The honeycomb structure may beconstructed from rigid or pliable integrity material depending ontailored use. It is seen that an expansion ability will be required insome embodiments so as pressure will not increase with phase change.

In FIG. 2 there is illustrated a sound absorbing device 1 of FIG. 1sandwiched between a series of layers of material. The sound absorbingdevice 1 of FIG. 2 comprises the sheet 2 of thixotropic material formedfrom thixotropic epoxy resin which is known to those skilled in the artin soundproofing and structural sealants for building constructionsandwiched between adhesive seal layers 30,31. The outer adhesive seallayers 30,31 seal the voids of the cells 5 of the honeycomb structure.An outer layer 33 and inner layer 34 are placed on the side of theadhesive seals 30,31 facing away from the sheet 2. The combination ofthe sheet 2 and layers 30,31,33,34 provide a sound absorbing device 1 ofthe present invention.

In FIG. 3 there is illustrated a further embodiment of the presentinvention where the device 1 is represented by a polymeric film having amultiplicity of honeycomb cells 5 and configured as a series of discs10,11,12. The series of discs 10,11,12 are stacked one on top of theother with decreasing diameters to provide a conical or substantiallyconical shape. An outer disc 10 has a diameter larger than inner disc11, which has a diameter larger than disc 12 when the central radiuspoint is taken at the centre of the device 1. FIG. 4 illustrates afurther embodiment of the device 1 of FIG. 3. An outer disc 20 and 21comprise a non-thixotropic material. The honeycomb structure may beconstructed from rigid or pliable integrity material depending ontailored use. It is seen that an expansion ability will be required insome embodiments so as pressure will not increase with phase change. Theinner disc 22 may aid in this expansion as well as the outercircumference material 21. The outer disc 20 is rigid so as to aidmanufacturing process and disposal within a housing. Between outer disc21 and an inner disc 22 is found a collection of honeycomb cells 5. Thehoneycomb cells 5 are interspersed with thixotropic material-filledstructures 7. In the illustrated embodiment, the honeycomb cells 5 arecomposed of a thixotropic material. FIG. 9 illustrates a furtherembodiment of the device 1 of FIG. 3. A disc 110 comprises anon-thixotropic material and having an outer rim 111. The honeycombstructure may be constructed from rigid or pliable integrity materialdepending on tailored use. The outer rim 111 is rigid so as to aidmanufacturing process and disposal within a housing. Within the outerrim 111 is found a collection of honeycomb cells 5. The honeycomb cells5 may be filled with thixotropic material. In the illustratedembodiment, the honeycomb cells 5 are composed of a thixotropicmaterial.

Referring to FIG. 5, these is illustrated a cellular scaffold formingpart of a sound absorbing device according to an alternative embodimentof the invention. The cellular scaffold 90 comprises a cylindrical tube91 having an exterior surface 92 and an interior surface 94 defining alumen 96 which is filled with a thixotropic material. In use, thecylindrical tube would be arranged within the sound absorbing device(not shown) such that external sound would enter the tube at a first end95 and travel through the tube to a second end 97. In doing so, thesound would cause some movement of the thixotropic material within thetube. The material at the walls of the tube would experience frictionand be exposed to a higher shear rate than the material at the centre ofthe tube. Thus, the thixotropic material adjacent the walls of the tubewould have greater sound absorbing capacity compared to the material atthe centre of the tube.

Referring to FIG. 6, these is illustrated an alternative cellularscaffold forming part of a sound absorbing device of the invention. Thecellular scaffold 90 comprises a series of cylindrical tubes 91 arrangedin an interleaving arrangement. In use, the cylindrical tubes wouldpreferably be arranged within the sound absorbing device (not shown)such that external sound would enter the tube at a first end 95 andtravel through the tube to a second end 97.

Referring to FIG. 7 and FIGS. 10A-10D, there is illustrated a series ofthixotropic material-containing cylindrical tubes 100, each tube beingcoiled back on itself, the series of curved tubes forming a U-shapedstructure 101. Each tube has two ends which, due to the U-shape, facethe same direction. In use, the U-shaped structure is disposed within asound absorbing device such that the ends of the tube face towardsincident sound, and the bend on the U-shaped structure is disposedtowards the user's ear. This structure directs the incoming sound backout to the environment. As such, most of the sound energy would bedirected back out to the environment even though the transmissionpercentage of sound in this part of the device 1 would be quite lowgiven that the sound energy passes through thixotropic material.

The housing 90 and substantially U-shaped structure 100 may be immersedin thixotropic material to further dampen and attenuate any sound notpassing through the tubes.

Referring to FIG. 8, there is illustrated a sound absorbing deviceaccording to the invention, in the form of a headphone set 60, andcomprising a headband 61 connecting ear coverings 62,63 together. Eachear covering 62, 63 includes a thixotropic material in the form of apolymeric sheet comprising a multiplicity of cellular pockets, whereinthe thixotropic material is located within the cellular pockets (notshown).

When a user is listening to music on the headphones 60, thixotropicmaterial located within the cellular pockets of the polymeric sheetwithin the cavities 66, 67 allows low intensity sounds like voices to beheard, while high intensity sounds like those from a pneumatic drill orjet engine reduced. As such, the user does not need to increase thevolume of the music to obviate the interfering external sounds. Thisadvantage of the sound absorbing device 1 of the invention reduces thedamage done to the hearing while maintaining the enjoyment of the musicbeing listened to.

Referring to FIGS. 11A-11C and FIGS. 12A-12D, there is illustrated afurther embodiment of cellular scaffold of the invention including theseries of thixotropic material-containing cylindrical tubes 100previously described with reference to FIG. 7 and FIG. 10. In theillustrated embodiments, each tube 121 is also coiled back on itself,the series of curved tubes 121 forming a substantially U-shapedstructure 120. Each tube has two ends which, due to the U-shape, facethe same direction. The series of substantially U-shaped structures 121are connected to and supported by a spine 122. The spine may be flexibleor rigid. The tubes 121 are connected to the spine 122 such that thetubes 121 point away from a plane X of the spine 122. As illustrated inFIG. 12A-12D, an additional series of cylindrical tubes 124 areconnected to and in fluid communication with the curved tubes 121. Thecylindrical tubes 124 lie perpendicular to the plane X of the spine 122.In FIGS. 13A-13D there is illustrated a further embodiment of a cellularscaffold 130 forming part of a sound absorbing device of the presentinvention. The cellular scaffold 130 comprises a plurality of tubes 90,as illustrated in FIG. 5, embedded in a base 131. The appearance of thescaffold 130 can be described as being like a “hedgehog”. As per FIG. 5,the plurality of tubes 90 comprises a cylindrical tube 91 having anexterior surface 92 and an interior surface 94 defining a lumen 96 whichis filled with a thixotropic material. In use, the cylindrical tubewould be arranged within the sound absorbing device (not shown) suchthat external sound would enter the tube at a first end 95 and travelthrough the tube to a second end embedded in the base 131. In doing so,the sound would cause some movement of the thixotropic material withinthe tube. The material at the walls of the tube would experiencefriction and be exposed to a higher shear rate than the material at thecentre of the tube. Thus, the thixotropic material adjacent the walls ofthe tube would have greater sound absorbing capacity compared to thematerial at the centre of the tube. FIG. 14 illustrates a furtherembodiment of the cellular scaffold 130 of FIG. 13. A second end 97 ofthe cylindrical tube 91 is exposed by an open-ended base 132. A seriesof substantially U-shaped cylinders 140 are embedded in the base 131,crossing from side 133 to the opposite side 134 of the base 131. Afurther series of substantially U-shaped cylinders 141 are embedded inthe base 131, crossing from side 135 to the opposite side 136 of thebase 131, and positioned such that the cylinders 141 are lying beneathcylinders 140.

Materials and Testing Method

A testing environment to ascertain the sound absorbing properties of thepresent invention is illustrated in FIG. 20. The testing environmentcomprises an anechoic chamber 200 with a decibel (dB) meter 201 on oneside of the chamber and a signal generator 202 on the other side of thechamber. A hearing protector 203, for example a pair of headphones, isplaced in the chamber 200 between the dB meter 201 and the signalgenerator 202. The hearing protector 203 is constructed such that thereis a join 204 running down the middle of the protector to accommodatethe easy insertion and removal of a cellular scaffold 210 (withreference to any of the cellular scaffolds of the Drawings andDescription) of the present invention inside the hearing protector 203.A cellular scaffold of the present invention is placed within theprotector 203 between the dB meter 201 and the signal generator 202within the chamber 200. A small speaker 205 is attached to the signalgenerator 202 and placed between the signal generator 202 and thecellular scaffold of the present invention. A sound sensor 206 (aDigitech QM 1592 class 2 professional sound level meter) is attached tothe dB meter 201 and placed between the dB meter 201 and the cellularscaffold of the present invention.

A measurement for background/ambient noise was first measured to ensurethat all sound from the signal generator 202 was being received by thedB meter sound sensor 206. The control used for the experiments is theindustry standard headphone ear protectors manufactured by 3M®, modeltype 1430C. The sound absorbing material of the control headphones weretested at frequencies indicated by the manufacturer, namely 125, 250,500, 1000, 2000, 4000 and 8000 dB. A reading for each frequency wasmeasured in triplicate and an average reading was calculated. Thereduction in the dB level achieved by the sound absorbing material ateach frequency was calculated by subtracting the measured dB level fromthe dB level measured when no sound absorbing material was present.

RESULTS

The embodiments described in FIGS. 1, 3, 11, 13 and 14 were tested. Theembodiments were assigned P1 (FIG. 11), P2 (FIG. 3), P4 (FIG. 13), P6(FIGS. 14) and P7 (FIG. 1). A summary of the results are presented belowin Table 1. Overall, the embodiments of the present invention which weretested demonstrated a significant advantage in hearing protection whencompared to the standard ear protection. The decibel is commonly used inacoustics to quantify sound levels relative to a 0 dB reference whichhas been defined as a sound pressure level of 0.0002 microbar. Thereference level is set at the typical threshold of perception of anaverage human and there are common comparisons used to illustratedifferent levels of sound pressure. As with other decibel figures,normally the ratio expressed is a power ratio (rather than a pressureratio).

The human ear has a large dynamic range in audio perception. The ratioof the sound intensity that causes permanent damage during shortexposure to the quietest sound that the ear can hear is greater than orequal to 1 trillion. Such large measurement ranges are convenientlyexpressed in logarithmic units: for example, the base-10 logarithm ofone trillion (10¹²) is 12, which is expressed as an audio level of 120dB.

TABLE 1 The dB Change achieved in seven embodiments (1-7) of the presentinvention and the industry standard 3M ® headphone at selectedfrequencies (Hertz (Hz)). 125 Hz 250 Hz 500 Hz 1000 Hz 2000 Hz 4000 Hz8000 Hz 3M 9.5 6.3 8.9 23.2 34.2 45.2 21.6 1 3.7 1.2 2.1 20.5 21.3 30 602 9.6 10.1 14.5 32.1 32.1 41 60 3 6.1 3.3 4.9 18.2 29.5 60 60 4 3.2 4.826.8 23.7 33.3 60 60 5 5.3 5.4 12.1 25.8 29 60 60 6 10.5 5.3 21.1 19.539.5 60 60 7 8.9 5.8 18 15.3 23.7 60 60

As illustrated in FIG. 15, there was significant sound absorption ratesachieved in the 4000-8000 Hz range as indicated by the superior changein decibel levels detected by the dB Meter. The cellular scaffold of thepresent invention excelled above 3M®'s sound absorbing material in the8000 Hz region by 40 dB. This type of protection is highly useful indentistry as the drills used in this profession function at thatfrequency level.

As illustrated in FIG. 16, there was significant sound absorption ratesachieved in the 125-2000 and 4000-8000 Hz range of the cellular scaffoldof the present invention when compared to the 3M® standard headphonesound absorbing material. The improved hearing protection in thesefrequency ranges would be an advantage to those in the constructionindustry and in professions where people are exposed to such highfrequency tones. In FIG. 17 there was significant sound absorption ratesachieved in the 250-1000 and 2000-8000 Hz range of the cellular scaffoldof the present invention when compared to the 3M® standard headphonesound absorbing material. The improved hearing protection in thesefrequency ranges would be an advantage to those exposed to such highfrequency tones.

In FIG. 18, it is clearly demonstrated that there was significant soundabsorption rates achieved in the 250-8000 Hz range of the cellularscaffold of the present invention when compared to the 3M® standardheadphone sound absorbing material. Such a significant improvement insound absorption within this frequency range would be a distinctadvantage to those working in construction surrounded by low frequencytones and those exposed to high frequency tones. FIG. 19 clearlydemonstrates that there was significant sound absorption rates achievedin the 250-500 and 2000-8000 Hz range of the cellular scaffold of thepresent invention when compared to the 3M® standard headphone soundabsorbing material.

The 250-1000 Hz range is the range involved in many industrial hardwareappliances and as such presents itself as a significant improvement inthe area of personal protection within the construction industry. The2000-8000 Hz range is an area of sound frequency which is related tohigh speed drills and electronic equipment. For example, dentists (anddental patients) are regularly at risk from high frequency drill soundsand as such, this improved protection is of significant value in thisprofession as a safety device. The results presented above clearlydemonstrate that the sound absorbing material contained in a cellularscaffold of the present invention achieves significant improvements insound absorption and hearing protection. Furthermore, the range offrequencies which the cellular scaffold of the present invention absorbssound allows the user to hear conversations while dampening the harmfuleffects of, for example, drilling noises and the like.

This technology can also be applied to other forms of hearingprotection. Individual earplugs can contain an insulating core thatcontains a sound absorbing material comprising a thixotropic material.Anechoic chambers can be constructed from panels of insulating materialthat would contain an internal structure of thixotropic material.

The invention is not limited to the embodiments hereinbefore describedbut may be varied in both construction and detail.

1. A sound absorbing device of the type adapted to cover the ears of auser and comprising a sound absorbing material contained within acontainer in the form of a cellular scaffold, wherein the soundabsorbing material comprises of a thixotropic material which is locatedin the cells of the cellular scaffold.
 2. A device as claimed in claim1, in which the sound absorbing material is enclosed within at least onecontainer, and wherein the container is expandable to allow an increasein volume of the sound absorbing material.
 3. A device as claimed inclaim 1 in which the cellular scaffold has a honeycomb structure.
 4. Adevice as claimed in claim 1 in which the cellular scaffold is in theform of a polymer film having cellular compartments or pockets, whereinthe sound absorbing material is located within the cellularcompartments.
 5. A device as claimed in claim 1 in which the cellularscaffold is in the form of at least one tube having first and secondends and a thixotropic material-containing lumen extending between theends.
 6. A device as claimed in claim 1 in which the cellular scaffoldis in the form of at least one tube having first and second ends and athixotropic material-containing lumen extending between the ends and inwhich the tube is disposed within the sound absorbing device such thatat least one end of the tube faces towards incident sound.
 7. A deviceas claimed in claim 1 in which the cellular scaffold is in the form ofat least one tube having first and second ends and a thixotropicmaterial-containing lumen extending between the ends and furthercomprises a plurality of interleaved tubes, or in which the cellularscaffold is in the form of at least one tube having first and secondends and a thixotropic material-containing lumen extending between theends and in which the tube is disposed within the sound absorbing devicesuch that at least one end of the tube faces towards incident sound andfurther comprises a plurality of interleaved tubes.
 8. A device asclaimed in claim 1 in which the cellular scaffold is in the form of atleast one tube having first and second ends and a thixotropicmaterial-containing lumen extending between the ends and in which the oreach tube is U-shaped and in which both ends of the tube face towardsincident sound.
 9. A device as claimed in claim 1 in which the cellularscaffold is in the form of at least one tube having first and secondends and a thixotropic material-containing lumen extending between theends and in which the or each tube is U-shaped and in which both ends ofthe tube face towards incident sound and in which the or each tubefurther comprise an additional tube disposed within the U-shaped tubebetween the first and second ends, wherein the additional tube has anopen end which faces towards the incident sound.
 10. A device as claimedin claim 1 in which the cellular scaffold is in the form of at least onetube having first and second ends and a thixotropic material-containinglumen extending between the ends and in which one end of each of aplurality of tubes is disposed within a base.
 11. A device as claimed inClaim 1 in which the cellular scaffold is in the form of at least onetube having first and second ends and a thixotropic material-containinglumen extending between the ends and in which one end of each of aplurality of tubes is disposed within a base and further comprising aplurality of substantially U-shaped tubes in which both ends of thetubes are disposed in the base.
 12. A device as claimed in claim 1 inwhich the cellular scaffold has a honeycomb structure and comprising atleast two layers of honeycomb structure arranged in a facingrelationship, wherein the at least two layers are disposed such that thecells of the honeycomb structure of a first layer are not in registerwith the cells of the honeycomb structure of the second layer.
 13. Adevice as claimed in claim 1 in which the cellular scaffold has ahoneycomb structure and comprising at least two layers of honeycombstructure arranged in a facing relationship, wherein the at least twolayers are disposed such that the cells of the honeycomb structure of afirst layer are not in register with the cells of the honeycombstructure of the second layer and further having three layers orientedat 120° apart.
 14. A device as claimed in claim 1 in which the cellularscaffold has a honeycomb structure and further comprising at least twolayers of honeycomb structure arranged in a facing relationship, whereinthe at least two layers are disposed such that the cells of thehoneycomb structure of a first layer are not in register with the cellsof the honeycomb structure of the second layer and in which the at leasttwo layers of honeycomb structure are separated by a sound insulatinglayer.
 15. A device as claimed in claim 1 in the form of an audio earbud, an audio headphone, an ear plug or a noise protection headphones.16. A device as claimed in claim 1 in the form of an audio or noiseprotection headphone, in which at least a part of the headphone isresiliently deformable such that deformation of the resilientlydeformable part applies shear to the thixotropic material. 17.(canceled)
 18. A device as claimed in claim 1 in the form of an audioear bud and/or ear plug in which the audio ear bud comprises a shellencasing a cavity, wherein the thixotropic material is disposed withinthe cavity, and wherein the shell is resiliently deformable such thatdeformation of the shell causes the application of shear to thethixotropic material.
 19. A sound absorbing device according to claim 1,wherein the thixotropic material is selected from the group comprisingstructured liquids, suspensions, emulsions, polymer solutions, aqueousiron oxide gels, vanadium pentoxide sols, starch pastes, pectin gels,flocculated paints, clays, soil suspensions, creams, drilling muds,flour doughs, flour suspensions, fibre greases, jellies, paints, honey,carbon-black suspensions, hydrophobically modified hydroxethylcellulose, non-associative cellulose water solutions, flocculatedpolymer latex suspension, rubber solutions, metal slushes, bentoniteclays, modified laponites, oils, lubricants, coal suspensions, xanthangums, organic bentonite, fumed silica, aluminum stearate, metal soap,castor oil derivatives or thixotropic epoxy resin, or combinationsthereof.
 20. A method of protecting an ear from high intensity noisecomprising the step of placing a sound absorbing device of the typeadapted to cover the ears of a user and comprising a sound absorbingmaterial contained within a container in the form of a cellularscaffold, wherein the sound absorbing material comprises of athixotropic material which is located in the cells of the cellularscaffold over the ear, wherein the thixotropic material has a restingviscosity which exhibits low resistance to passage of low intensitynoise, and wherein the thixotropic material decreases in viscosity inresponse to incident high intensity noises to thereby exhibit highresistance to the high intensity noise.
 21. A method as claimed in claim19 including an additional step of the user applying shear to thethixotropic material by shaking or deforming the sound absorbing device.